A Fin Field-Effect Transistor (FinFET) often includes a semiconductor fin, on which a gate dielectric and a gate electrode are formed. The gate dielectric and the gate electrode are formed on the sidewalls and the top surface of the semiconductor fin. The channel region of the FinFET thus includes a top surface portion and sidewall portions of the semiconductor fin.
Typically, the formation of the semiconductor fin includes forming Shallow Trench Isolation (STI) regions extending into a semiconductor substrate, with a portion (referred to as semiconductor strip hereinafter) of the semiconductor substrate being located between opposite sidewalls of the STI regions. The STI regions are then recessed, so that a top portion of the semiconductor strip forms a fin that protrudes above the top surfaces of the remaining STI regions. The conduction of the semiconductor fin is controlled by the gate electrode. The bottom portion of the semiconductor strip, however, is not controlled by the gate electrode, and leakage current occurs in this region when the respective FinFET is turned off.
Conventionally, to reduce the leakage current, an anti-punch-through implantation is performed to implant an impurity into the portion of the semiconductor strip that is lower than the top surfaces of the STI regions, hence forming an anti-punch-through region. The implanted impurity has a conductivity type opposite to the conductivity type of the source and drain regions of the FinFET. The anti-punch-through region has a high doping concentration, and hence has reduced carrier mobility than if the anti-punch-through region is not formed. The reduced carrier mobility results in the reduction in the leakage current.
Due to the high doping concentration in the anti-punch-through region, however, the risk of incurring Random Dopant Fluctuation (RDF) is increased. The risk is further worsened in recent generations of integrated circuits, in which the semiconductor fins and the underlying semiconductor strips become very thin, and hence the concentration of dopants in the anti-punch-through regions is increased. This results in the RDF variability problem to be more severe. In addition, the high doping concentration at the bottom of the fins results in the increase in the body effect, which degrades the sub-threshold slope of the resulting FinFETs.